(a) Field of the Invention
The present invention relates to a solid state imaging device and a method for manufacturing the same.
(b) Description of the Related Art
Solid state imaging devices such as an interline CCD image sensor have a structure shown in FIGS. 1A and 1B, wherein FIG. 1A is a top plan view of the imaging device and FIG. 1B is a sectional view taken along line Axe2x80x94A in FIG. 1A. A plurality of photodiodes 11 are arranged in a matrix on an N-type silicon substrate 18, and a vertical CCD 12 is provided for each column of the photodiodes 11 for reading signal charge from each photodiode 11 through a charge transfer channel 14 and transferring the same to a horizontal CCD not shown. Each of the photodiodes 11 is separated from adjacent photodiodes by a P+-type isolation region 13. If the imaging device is not of a CCD type, the CCD 12 in FIG. 1A is replaced by drain regions of MOSFETs.
In operation of the solid state imaging device as described above, signal light incident onto the photodiode 11 is converted to signal charge by a photoelectric conversion, accumulated for a specified interval, then read out from the photodiode 11 to the vertical CCD 12 by applying a transfer voltage to a transfer gate provided above the charge transfer channel 14.
After the signal charge is transferred from the photodiode 11, the N-type substrate 18 is applied with a high voltage pulse for withdrawing electrons from the photodiodes 11, thereby effecting a xe2x80x9csubstrate shutterxe2x80x9d. The substrate shutter provides control of amount of the stored charge in the photodiode 11.
In general, it is preferable that the photodiode have a higher photo-sensitivity and a larger capacity for storing the signal charge. In addition, there is an increasing demand for reduction of each pixel area or photoelectric conversion area to decrease the total area and the cost of the solid state imaging device as well as to increase the number of pixels per unit area. The reduction of the pixel area is generally implemented by reduction of the area occupied by the photodiode 11.
Reduction of the area occupied by the photodiode involves several problems. First, a three-dimensional effect occurs wherein the influence by the isolation region increases, resulting in that the depth of the photodiode cannot be evaluated in a single dimension. This in turn results in reduction of the stored charge compared to the expected stored charge which is calculated from the ratio (area ratio) of the photodiode area to the total area.
Second, since the photodiodes are subjected to depletion during the substrate shutter by depleting the isolation region, the area of which is relatively increased by reduction of the photodiode area, the pulse voltage necessary for the substrate shutter is increased. In this case, if the stored charge is to be increased by raising the impurity concentration of the charge storing area of the photodiode, the higher pulse voltage for the substrate shutter may exceed the power supply voltage of a portable device, or may increase power dissipation in the portable device. Thus, increase of the stored charge in the photodiodes is incompatible with the reduction of the pulse voltage for the substrate shutter.
In view of the above, it is an object of the present invention to provide a solid state imaging device capable of increasing stored charge in the photodiodes and suppressing the increase of the pulse voltage for the substrate shutter.
It is another object of the present invention to provide a method for fabricating the solid state imaging device as described above.
The present invention provides a solid state imaging device including a semiconductor substrate having a first conductivity type, a first layer having a second conductivity type opposite to the first conductivity type, a charge storing area formed on the first layer and having the first conductivity, a second layer formed on the charge storing layer and having the second conductivity type, the charge storing area and the second layer forming a photodiode, a charge transfer channel formed adjacent to the charge storing area, an isolation region having the second conductivity, the charge storing area being surrounded by the isolation region except for a side adjacent to the charge transfer channel, and an additional implant area subjected to implantation of impurity ions and disposed in a peripheral region of the charge storing area.
The present invention also provides a method for forming a solid state imaging device including a semiconductor substrate having a first conductivity type, a first layer having a second conductivity type opposite to the first conductivity type, a charge storing area formed on the first layer and having the first conductivity, a second layer formed on the charge storing layer and having the second conductivity type, the charge storing area and the second layer forming a photodiode, a charge transfer channel formed adjacent to the charge storing area, an isolation region having the second conductivity, the charge storing area being surrounded by the isolation region except for a side adjacent to the charge transfer channel, the method comprising the step of forming an additional implant area by implanting additional impurity ions having the first conductivity type into at least one of a peripheral region of the charge storing area and the isolation region.
In accordance with the solid state imaging device of the present invention and fabricated by the method of the present invention, the charge stored in the charge storing area can be increased by the additional implant area substantially without increasing the pulse voltage for the substrate shutter.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.